1. Field of the Invention
The present invention relates to a disk unit, more particularly, to a compensation for an offtrack occurring in various magnetic disk units using a servo surface servo system.
2. Description of the Related Art
Increasing the storage capacity of a magnetic disk unit typically requires an increase in the number of disk media arranged on a disk element, the number of heads, and track density.
When the storage media and track density are increased, a data head that is controlled ontrack on a data surface according to servo information recorded on a servo surface will easily slip off a track if the environment of the unit changes, in particular, if ambient temperature changes from low to high, or from high to low. If the data head slips off a track, no data will be read.
To prevent such data head offtrack due to an ambient temperature change, an offset (an offtrack amount) of the data head with respect to an ontrack position is found at regular intervals by reading servo information recorded on a disk surface.
When accessing a track for reading or writing data, an offset compensation operation is carried out to shift a selected data head to remove an offset.
Even if a positional deviation occurs between a servo surface and a data surface due to a change in ambient temperature, the above technique may control the data head to an ontrack position.
Conventionally, the offtrack has been compensated by writing servo information not only on the servo surface but also on the data surfaces.
However, the offtrack amount undergoes a change with time, and the amount of deviation of the data head position is not often the same as the amount of deviation of the servo head position. In such a case, the offtrack must be compensated while compensating the servo surface by reading servo information on the data surface or compensating the data surface by reading servo information on the servo surface.
Therefore, the conventional offtrack compensation requires a considerable period of time and, besides, it is difficult to precisely compensate the offtrack.
Also, the measured offtrack amount undergoes a change due to position control errors that develop in a random fashion when it is being measured. Therefore, measurement must be taken several times and must be averaged to obtain an effective result of measurement. This requires an extended period of time for measurement.
Furthermore, when offtrack compensation is performed, if a quantity of offset is large, a head may reach the vicinity of the middle between a servo track and an adjacent servo track.
In this case, it is difficult for the head to perform stable tracking. In this connection, if the measured offtrack amount takes on a value that is more than half a width of a track, it is more likely that the disk unit has a mechanical defect.
Note, the problems in the prior art will be explained later in detail in contrast with the preferred embodiments of the present invention.